1. Field
Embodiments of present inventive concepts relate to an apparatus and a method for heating plastic containers including, for example, plastic preforms.
2. Description of the Related Art
It is common practice during the manufacture of plastic containers to heat the containers when they are plastic preforms and to subsequently expand them into the desired shape in a moulding process such as, for example, a stretch blow moulding process. For this purpose, infrared ovens through which the plastic preforms run and in which the plastic preforms are heated, have been used. Microwave ovens have also been used, in which the plastic preforms are heated by microwaves.
Examples of a heating apparatus for plastic preforms is known from DE 10 2007 022 386 A1. The disclosure therein is herein, in its entirety, incorporated by reference. A problem in connection with the heating of plastic preforms for the stretch blow moulding process using microwaves in a cavity resonator is to heat the plastic preforms uniformly. A method is known from the above-mentioned document as well as from DE 10 1008 024 108, herein incorporated by reference in its entirety, in which an attempt is made to uniformly heat the longitudinal sides of the plastic preforms. However, problems arise here in the dome areas of such plastic preforms, i.e. at the lower ends thereof.
As a result of the behaviour of electromagnetic fields on interfaces between two different dielectrics, the electric field is markedly stronger in the areas of the transitions parallel to the electric field than they are in the areas that are perpendicular thereto.
However, since the angle of the electric field E changes due to laws of refraction, the strength of the electric field can change. As a result, E vectors which enter approximately parallel to the interface will remain approximately constant, and in the case of vectors that enter at an acute angle, the amount will abruptly decrease. Therefore, the side of the plastic preform will be heated comparatively intensely and the dome will remain cold.
In connection with this, so-called single mode cavity resonators are in some cases used. Due to their size and the frequency radiated by the magnetron, these only allow a so-called H10 mode because no other resonances can form due to the propagation laws and the interface lengths. Since the E field lines are vertical on the metal surfaces (because all non-vertical components would flow off via the metallic conductor), the field of the H10 mode is vertical between the two parallel walls of the resonator. For the interface length λc (below which no wave can form in the resonator), the relationship λc=2 a applies, wherein a is the length of the broader side (e.g. of a waveguide).
This means that it is considerably more difficult to heat the plastic preform in the area of the dome, in particular around the injection point, than it is at the longitudinal sides.
One possible solution to this problem is proposed in DE 10 2007 022 386, herein incorporated by reference in its entirety, which describes a reflector element. Such a metallic reflector element can lead to distortion of the electrical field in the vicinity of the conducting surface and also to a rapid increase in case of a proper selection of geometry. This local increase can enable an improved heating of the dome region. On the other hand, such “reflector elements” also have disadvantages. Thus, for example, they may get contaminated and will therefore have to be cleaned using an additional cleaning unit such as, for example, a permanently installed brush.